ABSTRACT: The balance of forces which effect lung tissue hydration has been investigated in the past by assuming that the lung behaves as a single homogenous tissue which can be described in terms of a single tissue hydrostatic and osmotic pressure. Obviously, since capillary pressure changes at each lung height, then tissue hydrostatic and osmotic pressure may also vary in a predictable fashion at different lung levels. We also know from the tendency of interstitial edema fluid to collect in perivascular and peribronchial cuffs that horizontal interstitial pressure gradients exist which favor movement of fluid from the parenchyma to the interstitium surrounding larger pulmonary vessels. The major objective of this research project is to investigate the vertical and centeripetal pressure gradients in lung tissue in both normal and edematous lungs using chronically implanted pressure devices (modified capsules) and alveolar fluid absorptive pressures. In addition, the effects of different hydration states on protein exclusion by the interstitial gel matrix will be studied. From an analysis of the measured pressures and exclusion properties of the interstitium, and the kinetics of lymphatic proteins it will be possible to describe the forces which move fluid and proteins across the pulmonary capillaries at each lung level and within the interstitial space. In other experiments outlined in this proposal we will investigate the differences in vascular permeability which exist between arteriolar and venular ends of the pulmonary vessels, between bronchial and pulmonary vessels, and the effect of electrostatic change on macromolecular transport, because these factors may also influence regional fluid balance within the lung.